Can you think of an example of an intact protein molecule being taken up by a free-living bacterium?
We jumped the gun and departed from the usual format by sending this question in advance to a few people. This netted us some intriguing thoughts from Jon Beckwith, Winfried Boos, Ian, Booth, and Andrew Wright, plus notes from a few others who told us they couldn't come up with much. Now it's your turn. Please don’t be deterred from adding your ideas and comments.
Here’s what we have so far:
- Bacteriocins. Many are small peptides, but some are as big as 20 kDa, so at least these qualify as respectable proteins. And they certainly penetrate, or else they wouldn't work on such targets as DNA, rRNA, and tRNA. The uptake mechanism is known in some cases to involve porins.
- Phage proteins. DNA phages inject so-called “pilot proteins” needed for the translocation of their DNA. In the classic Hershey-Chase experiment, a small proportion of the 35S-labeled proteins of the original phage was associated with the bacteria after infection. This didn't bother Hershey and Chase but later turned out to be of interest. However, does “injection” by a phage count?
- Periplasmic proteins such as Maltose Binding Protein can penetrate into the periplasm when added in solution to E.coli cells permeabilized with calcium. But they are not known to enter the cytoplasm, so this doesn't count, does it?
- Other. Andrew Wright wrote: Another "artificial system" is REMI (restriction enzyme mediated insertion) where restriction enzymes are introduced into cells (yeast and Dictystelium) by electroporation. I don't know if this works with bacteria but my guess is that it should. Of course electroporation cuvettes are not floating around in the environment but other conditions in nature either now or in the distant past might have mimicked this. I imagine many types of proteins could be introduced in the same way — if attached to DNA or some other macromolecule.
Why would we want to know? This qualifies as another Talmudic Question itself. But not to prolong matters, here are some of our answers:
- Organelles, such as mitochondria and chloroplasts, take up proteins that are encoded by the nuclear genome and synthesized on ribosomes in the cytoplasm. When and how did they acquire this skill? Should we address the question to their ancestral relatives, the rickettsiae and the cyanobacteria? And how about bacterial endosymbionts?
- Could proteins involved in DNA repair/mutagenesis when taken up from the environment play a role in bacterial evolution?
- Could soluble proteins be taken up to act as signaling agents between species?